You have a couple of options here. You can either prepare the disk; so OSX knows to not index them, configure it from System Preferences or disable it completely.

All external Disks

The secret is that all external volumes are mounted into /Volumes

So open up Terminal in an account with Admin privileges. Enter “open /Volumes” and return. This will open the folder in the Finder. Now open your Spotlight System prefs pane and click on “Privacy”. Drag the icon at the top of the Folder window “Volumes” and drop it into the list. You’ve now added the /Volumes folder to the places not to index and now inserting a USB should not cause it to be indexed.

You could also do this without GUI:

create an empty file on the root of the volumes: .metadata_never_index

Per Disk

If you like the indexing of external disk, and just want to disable it for a single disk: